1. Field of the Invention
The present invention relates to a thin-film solid oxide fuel cell using a metal support. More specifically, the present invention relates to a cell body for a fuel cell, which is capable of constituting a cell element (a single cell) and a gas flow path at any arbitrary position on the metal support, and capable of obtaining a fuel cell that is excellent in output density per volume at low cost. Moreover, the present invention relates to a method of manufacturing the cell body and to a solid oxide fuel cell stack using such a cell body.
2. Description of the Related Art
In recent years, fuel cells have gained attention as a clean energy source, which is highly efficient in electric power generation, generates little poisonous exhaust gas, and is friendly to the environment.
From among a variety of fuel cells, a solid oxide fuel cell (hereinafter, referred to as “SOFC”) uses, as an electrolyte, an oxide ion conductive solid electrolyte such as yttria-stabilized zirconia (hereinafter, referred to as “YSZ”). The SOFC is a fuel cell in which electrodes with permeated gases are provided on both surfaces, and fuel gas such as hydrogen and hydrocarbon is supplied to one electrode and oxygen gas or air is supplied to the other electrode with the solid electrolyte taken as a partition wall, thus generating electric power.
The operation temperature of the conventional SOFC is as high as approximately 1,000° C. because ion conductivity of a solid electrolyte layer thereof is insufficient if the temperature of the solid electrolyte layer is not high. Therefore, the conventional SOFC has had problems in safety, reliability in the operation thereof, and the high cost of high temperature resistant materials, and the like.
Accordingly, realization of the SOFC operation at a lower temperature has been taken up as an important subject. Progress has been made in the development of a solid electrolyte material exhibiting high ion conductivity even at low temperature, and in the development of a thin-film electrolyte SOFC using a thin solid electrolyte layer and reducing overvoltage of the solid electrolyte layer, even at low temperature.
In the development of such solid electrolyte materials exhibiting high ion conductivity at low temperature, for example, a solid electrolyte material using a perovskite oxide such as LaSrGaMgO3 and the like has been developed, and a material exhibiting equivalent ion conductivity at 600° C. to that of the above-described YSZ operated at 1,000° C. has been proposed.
Meanwhile, a development of the thin-film electrolyte SOFC has been introduced in D. Ghosh et al.; Electrochemical Society Proceedings, Vol. 99-19. In this literature, a fuel electrode material is used as a base material, and an electrolyte layer having a thickness no thinner than gap hollows on the surface of the fuel electrode material is printed and sintered thereon, and thus the thin-film electrolyte SOFC is prepared, whereby the thinning of the solid electrolyte layer is realized. However, in this structure, it is extremely difficult to form the electrolyte layer so that it is not more than 5 μm because the thickness of the electrolyte layer depends upon the hollows of the sintered body forming the solid electrolyte layer.
Therefore, a new deposition method such as an Electrochemical Vapor Deposition method (EVD method) has been proposed for the surface of a porous material (S. C. Singhal,; Electrochemical Society Proceedings, Vol. 97-18). However, this method is not practical due to the extremely slow deposition rate thereof.
A subject of the thin-film electrolyte SOFC is to form a dense thin-film solid electrolyte layer, which is not permeated with gases, on a porous electrode surface having numerous open gaps. Accordingly, Japanese Patent Application Laid-Open No. H6-88199 (1994) proposes a method of manufacturing a thin-film-coated porous metal material as will be described below. In this method, a predetermined thin film is coated onto a metal support composed of a sintered body that is obtained by dispersing, in a metal matrix, a combustible material such as carbon fiber and crystalline cellulose, and a soluble material such as aluminum and soluble glass fiber or the like. Then, a dispersed phase is removed by heating or chemical treatment, and thus the metal support is made to be porous.